Halliday & Matthiessen (2014: 390-1):
We have assumed so far that the Head of the univariate structure is also always the Thing, in the multivariate structure. But this is not so. There is always a Head in the nominal group (unless it is ‘branched’, like one brown in one blue eye and one brown [Ø]); but there may be no Thing. It is quite normal to have Numerative or Deictic as Head, as in Figure 6-9:
